A prior-art technique of measuring torsional vibration of a rotator is disclosed in Japanese Patent Application Publication Tokkai Hei-7-5056 and U.S. Pat. No. 5,438,882, for example. This system includes a pair of optical detectors with convergence lenses, a controller for converting the output of the optical detectors into digital data, an oscilloscope for displaying the output wave shapes of the optical detectors, a computer for analyzing the digital data, a display device for displaying the analyzed results and reflective mark bands attached on the rotator.
The reflective mark bands are wound around the rotator to be measured. The reflective mark bands have a plurality of lines or marks parallel to the axial direction, which are arranged along the peripheral direction with equal pitches “d”. The lines and the marks are reflective, and the other portion of the mark bands is non-reflective. Each of the pair of optical detectors with the convergence lenses are aligned with corresponding reflective mark bands of the pair of reflective mark bands.
In operation of this prior-art system, when a rotator of radius of R which is rotating with an angular velocity of Ω, the light reflected on the reflective mark band is converged by the convergence lenses and detected by the optical detectors. As a result, pulse signals with time intervals of τ=d/(RΩ) are detected by the oscilloscope which displays the output wave shape of the optical detectors.
When there is no torsional vibration, the phase difference of the pulse signals is constant. On the other hand, when there is a torsional vibration, the phase difference of the pulse signals is not constant, and the phase difference vibrates. The pulse signals are digitized in order to determine the fluctuation of the phase difference. Then, the fluctuation of the phase difference is determined by digital analysis using a computer. The torsional vibration frequency is derived from the fluctuation of the phase difference, and the torsional vibration frequency is shown on the display device.
In the prior-art system described above, when the rotator to be measured has axial vibration or non-torsional vibration, the rising and dropping times and the signal amplitude of the pulse signal change due to the axial vibration. Therefore, idealistic pulse signals can hardly be obtained, and then, torsional vibration and rotational speed cannot be accurately measured.
The present invention is to solve the problem described above. An object of the present invention is to present a torsional vibration measuring instrument which can measure torsional vibration of a rotator with high precision even when the rotational axis of the rotator to be measured has an axial vibration.